The invention relates to the field of electric machinery design generally, and more particularly to providing a cryosorption pump for perpetuating a thermal insulating vacuum in the superconducting rotor of an electric machine, such as a turbogenerator.
Recent developments in the field of electric machinery design suggest providing a superconducting exciter winding for the rotor of an electric machine, for example a turbogenerator, that is generally cooled with liquid helium. To limit heat transfer from the outside to this refrigerated winding, the Winding is appropriately surrounded by an insulating vacuum, which in general should be less than 10.sup. -6 mbar. Because of the cooling with liquid helium, all gas components with the exception of helium are indeed bound to helium-cold surface portions of the rotor due to a cryopumping effect. However, the insulating vacuum is diminished by helium leaks of the helium-carrying rotor parts, in particular at solder and weld joints. For the helium-carrying parts of the machine, when the machine is operated for more than one year, the leakage rate should be less than 10.sup. -10 mbar liters each second if it is not to be necessary to pump the helium out of the insulating vacuum. However, such low leakage rates can be achieved only at very high cost of engineering in the selection of materials and in the production process. Typically, the leakage rates for the helium-carrying parts are approximately 10.sup.-8 mbar liters per second. With long operating periods of several years, as required for turbogenerators, an impermissible pressure rise of the insulating vacuum results. For example, at this typical leakage rate, in a vacuum volume of 1000 liters, the pressure increases to 10.sup.-4 mbar in less than a year. Hence a pump is needed which can pump off the helium reliably and largely without maintenance over the period referred to.
If, for this purpose, an external pump is used, then, for connection to the rotating vacuum chambers of the rotor in a so-called connection head, a transition from fixed to rotating parts of a pump conduit with an appropriate vacuum rotary bushing in the connection head is necessary. The pump conduit extends through a corotating transition part which, in the form of a shaft end piece of the rotor, protrudes into the connection head. This transition part on the side of the rotor away from the drive serves in particular to receive vacuuminsulated coolant lines between the connection head and the actual rotor or coil body (see, for example, "Development of a Helium Transfer Coupling for a Superconducting Generator Rotor" by B.B. GambIe in Advances in Cryogenic Engineering, vol. 23, 1978, pages 125 to 129 and pages 132 to 139). The vacuum rotary bushing in the connection head can be sealed in particular by a so-called ferrofluid seal (See, for example, German Offenlegungsschrift 20 34 213). The difficulty here is, however, that the suction capacity of the pump is considerably reduced by the conduction values of the relatively long connecting lines and by the vacuum bushing. Besides, the vacuum rotary bushing to be sealed with ferrofluid is especially sensitive to shaft vibrations, and thus is a possible source of malfunction in the power plant operation.
Another known solution is to install a co-rotating cryosorption pump in the rotor which has no mechanical connection to the surrounding. This, therefore, eliminates any requirement for a separate pump conduit to the outside with a critical vacuum rotary bushing. Such an evacuating device with an ionization getter pump is known from the German Auslegeschrift 28 26 501.
To maintain a thermo-insulating vacuum in the rotor of a turbogenerator with a superconducting exciter winding, there may be used advantageously also a cryosorption pump as described, for example, in the German Offenlegungsschrift 28 06 576. The respective pump principle is known generally (see, for example, M Wutz: "Theory and Practice of Vacuum Technology" (in German), Brunswick, 1965, in particular pages 213 to 222). In this known generator, the pump is disposed in the active part on the coil former of the rotor and is connected with the vacuum chambers of the rotor by means of ports. For cooling the rotor, the pump is thermally coupled to cold helium which is supplied to the exciter winding via a corresponding supply line. As sorbents are used, in particular, zeolites or activated carbon, these sorbents may be bound either loosely or fixed to metallic substrates. Because of the structure of the known cryosorption pump in the region of the exciter winding of the rotor body being situated radially far out from its axis, only a limited space is available for this pump, which is relatively complicated in its design. Besides, after assembly of the rotor, this pump cannot readily be removed again, making it extremely, difficult to service.
The problem presented by known devices then is to improve the design of cryosorption pumps in the sense that the pump should permit a relatively simple construction, not requiring any special measures for the realization of the rotor coil form carrying the exciter winding.